Where can I access Sketchfab upload (and other tabs)
Make sure you are in Object Mode, not Edit Mode, nor Texture Paint mode in the menu at the bottom of the 3D View Window Header. The tab will not show up unless you are in object mode.

Also in User Preferences > Addons make sure to expand the Sketchfab Addon tab and check the tab name written there. If changed the Title of the tab in the tool panel may differ.